A lumbar motion segment is considered to be unstable when it exhibits abnormal movement. This movement can be abnormal in quality (abnormal coupling patterns) or in quantity (abnormal increased motion). This instability can be symptomatic or asymptomatic, depending on the demands made on the motion segment. Pain is a signal of impending or actual tissue damage, and when present it indicates that a certain mechanical threshold has been reached or transgressed. Repeated transgressions will damage the stabilizing structures beyond physiologic repair, thus putting abnormal demands on secondary restraints. Radiographic study with dynamic views obtained in the frontal and lateral planes identify unstable states in the clinical environment.